The invention relates to the general field of telecommunications. The invention relates in particular to a routing protocol for an ad hoc network enabling communication to take place between vehicles.
For several years, a large amount of work has been done concerning intelligent transport systems (ITS). In particular, proposals have been made to provide an ad hoc network enabling communication to be provided between vehicles by means of on-board ITS stations. Applications for communication between vehicles include in particular exchanging information about road safety, returning vehicle data for preventative maintenance, returning vehicle utilization information for the purpose of billing insurance based on use, returning vehicle path data for paying an ecotax or city toll charges, etc.
Given the specific features of such applications, proposals have been made to base the operation of such a network on technology known as geonetworking, in which addressing and routing are based on the geographical position(s) of the destination(s), in contrast to a conventional network which makes use of network addresses, e.g. Internet protocol (IP) addresses. The European Telecommunications Standard Institute (ETSI) gives the following definition of geonetworking: “A network service that uses geographical positions and provides ad hoc communication without the need for a communications coordinating infrastructure”.
ETSI's technical specifications document TS 102 636-4-1 describes a stack of protocols known as GeoNetworking. One of the data structures used in those protocols is referred to as the location table.
Each entry in the location table corresponds to an ITS station, either for which there is an ongoing location request, or else for which the location is known. Each entry has a lifetime, which is set at 20 seconds by default. When this lifetime is reached, the entry is deleted from the table.
An entry contains the following information: the address (in the GeoNetworking network sense) of the ITS station in question; the address of the station at data link level; the type of the station (vehicle or roadside), the station's position vector (position, speed, heading, position time stamp, position accuracy, speed accuracy, heading accuracy), a Boolean flag indicating whether a location request is or is not in progress, a Boolean flag indicating whether the ITS station is in the direct vicinity, and the sequence number of the packet most recently received from the station.
When a source ITS station needs to send a message to a destination ITS station, it needs to know the geographical location of the destination station. If this information is not available in the location table, the source station launches a location request. This propagates in compliance with the protocol in use until it reaches the destination station. That station then responds to the source station by supplying it with its own location.
Furthermore, each ITS station broadcasts its position on a regular basis to its neighbors. By default, the period for this broadcasting is set at 3 seconds.
Each entry in the location table is created and/or updated as a function of service requests from higher layers, and as a function of communications with the other ITS stations. Thus, by way of example, a request to send a data message to an ITS station that does not yet have a corresponding entry in the location table will cause a location request to be started (which will take place using the above-described mechanism), and will cause an entry to be created in the location table for the station in question, the corresponding Boolean flag specifying that a location request is in progress. When the response to the location request is received, the flag is set to indicate that the location has been received, and the position vector of the station is updated.
Likewise, when a packet is received from a neighboring ITS station, or when a packet transmitted by a neighboring ITS station is received for transferring to another ITS station, the entry in the location table corresponding to the ITS station that transmitted the packet is updated, with the previously existing position vector being replaced by the position vector contained in the packet, if that vector is more recent.
Updating an entry in the location table resets the associated lifetime to zero.
The GeoNetworking routing algorithms rely on the content of the location table in order to route the packets from ITS station to ITS station.
The content of the location table is relatively local to the ITS station in question, since it is derived mainly from exchanges with neighboring ITS stations. Furthermore, in situations in which the vehicles carrying the ITS stations have paths that have little correlation (typically away from motorways and main roads), the entries describing neighboring ITS stations disappear quickly from the location table, since the vehicles in question move apart from one another.
The quantity and the pertinence of the information contained in the location table are thus limited. There is therefore a need to improve the routing of a data packet in a network based on geonetworking.